Aes is the central regulator of the Escherichia coli maltose system. Aes catalyzes hydrolysis of p-nitrophenyl esters of fatty acids, preferring substrates with short (less than 8 in length) acyl chains. Aes binds to the maltose operon regulator, MalT, and inhibits MalT transcriptional activation activity in competition with its inducer, maltotriose. Aes also binds to alpha-galactosidase; this interaction stimulates Aes esterase activity and inhibits alpha-galactosidase, suggesting that Aes may have a physiological role in regulation of metabolism of carbohydrates. The enzyme is monomeric. The catalytic triad is predicted to comprise Ser165, Asp262, and His292. In the presented crystal structure of Acetyl Esterase from Salmonella typhimurium active site Ser-165 is phosphorylated.